(a) Field of the Invention
This invention relates to acid-curable resin binder compositions useful in a no-bake foundry process. More particularly, the invention relates to a solution containing a phenolformaldehyde resole acid-curing resin, a furan resin or their combination in the form of a prepolymer which have incorporated therein a fluoride ion.
(b) Description of the Prior Art
Foundry cores and molds used in making metal castings are normally prepared from a composition including sand or other refractory material and a curable or polymerization binder coated on the refractory particles. The purpose of this binder coating is to permit the mixture to be hardened after it is first shaped or molded into a desired form. Shaping of the composition, which usually comprises a major amount of sand and a minor amount of binder, is accomplished through ramming, blowing, or otherwise introducing the mixture into a pattern to thereby assume the shape defined by the adjacent surfaces of the pattern. Then, by using a catalyst or polymerization accelerator introduced before or after the sand mix has been introduced into the pattern, and/or by using heat, the binder is caused to cure, thereby converting the shaped foundry mix into a hard, solid foundry core. This curing is usually accomplished either in the original pattern, in a gassing chamber, or in a holding pattern. Commonly used binders include such materials as phenolic resole resins, urea-formaldehyde resins, furfural alcohol modified urea-formaldehyde resins, furan resins, drying oils and urethane oils.
Generally speaking, two basic techniques exist in the art for effecting a cure once the sand-binder mixture is shaped. The first of these techniques, the elevated temperature method, involves the use of a heat-curable resin system wherein heat is used to effect hardening of the binder. The second technique is known in the art as the "no bake" or "cold-setting" process. As its name implies, the latter process is carried out at room temperature or slightly above; i.e., 5.degree.-50.degree. C. and more often between 15.degree.-35.degree. C.
The first technique, the thermosetting process, allows for high speed automatic core and pattern production (i.e., hardening in 10-45 seconds). However, it has many problems attendant with it. For example, the addition of heat is an added expense to the system as a whole. The high temperatures required, which usually are from about 175.degree. C.-260.degree. C., necessitate the use of special metal patterns adapted to withstand these temperatures and thus are expensive to make and difficult to maintain. Furthermore, automatic core machinery used in the art is difficult to maintain at the elevated temperatures. Should the machinery break down, it is often necessary to let the machinery cool in order to make repairs. Cooling time obviously causes long and expensive delays in production.
The second technique, the "no-bake" or cold-setting process was developed to alleviate the problems attendant with the hot-box process of curing foundry cores. Through the use of the low temperatures of the no-bake or cold-setting process, the problems of the hot-box process are virtually eliminated. However, because of the use of low temperatures, useable strengths are generally achieved only after about 15 minutes and usually closer to 60 minutes cure after the no-bake resin is mixed with sand and a catalyst, compared to 10-45 seconds when using a thermosetting process. In addition, the resins heretofore developed by the art which may be cured at these relatively low temperatures may suffer from lack of tensile strength, short bench life of foundry mixes, etc.
It is quite evident that there has been a need in the foundry core art for a system of making cores which is rapid, which at the same time imparts useable strengths to the cores without the need for high temperatures, and which forms a foundry core with good shake-out and collapsibility characteristics without the foregoing disadvantages.
Progress had been made in overcoming the problems associated with the no-bake resin systems, some of which have been described in the patent literature; e.g., U.S. Pat. Nos. 3,525,379; 3,549,584; and 3,644,274. U.S. Pat. No. 3,644,274 to Maurukas describes a process for the acid-catalyzed polymerization of a fusible furan resin by incorporating a source of fluoride ion in the acid catalyst and the process for forming foundry cores or molds from sand using this new resin system. More particularly, a new acid catalyst system is used whereby a strong acid is admixed with a fluoride ion and this mixture is used to cause polymerization of the resin. Alternatively, the fluoride ion can be mixed with the sand and with the subsequent addition of the acid component, the fluoride ion is available as part of the catalyst system. The fluoride ion/acid catalyst development described in U.S. Pat. No. 3,644,274 has represented an improvement in the no-bake foundry systems, particularly the furan-type resins and conventional phenolic resole resins, both of which have been used on a commercial basis. While the addition of fluoride as part of the catalyst system as described in U.S. Pat. No. 3,644,274 improves the tensile strength of the foundry cores and molds, it has been found that this catalyst system containing fluoride ion often releases a pungent odor due to the presence and/or release of hydrofluoric acid which is undesirable both from an environmental and health of foundry workers standpoint. With increasingly tight air pollution laws and Occupational Safety and Health Act (OSHA) laws there is concern over the use of these fluoride-containing catalysts. Thus, there is a need to find the means to reduce these problems without sacrificing the beneficial properties of the prior art resin systems.